Unified receiver for layered and hierarchical modulation systems

ABSTRACT

A satellite receiver includes a down converter for providing a received signal and a demodulator having at least two demodulation modes for demodulating the received signal, wherein one demodulation mode is hierarchical demodulation and another demodulation mode is layered demodulation.

This application claims the benefit, under 35 U.S.C. § 365 ofInternational Application PCT/US04/013734, filed Apr. 24, 2004, whichwas published in accordance with PCT Article 21(2) on Dec. 2, 2004 inEnglish and which claims the benefit of United States provisional patentapplication No. 60/471,167, filed May 16, 2003.

BACKGROUND OF THE INVENTION

The present invention generally relates to communications systems and,more particularly, to satellite-based communications systems.

As described in U.S. Pat. No. 5,966,412 issued Oct. 12, 1999 toRamaswamy, hierarchical modulation can be used in a satellite system asa way to continue to support existing legacy receivers yet also providea growth path for offering new services. In other words, abackward-compatible hierarchical modulation based satellite systempermits additional features, or services, to be added to the systemwithout requiring existing users to buy new satellite receivers. In ahierarchical modulation based communications system, at least twosignals, e.g., an upper layer (UL) signal and a lower layer (LL) signal,are added together to generate a synchronously modulated satellitesignal for transmission. In the context of a satellite-basedcommunications system that provides backward compatibility, the LLsignal provides additional services, while the UL signal provides thelegacy services, i.e., the UL signal is, in effect, the same signal thatwas transmitted before—thus, the satellite transmission signal cancontinue to evolve with no impact to users with legacy receivers. Assuch, a user who already has a legacy receiver can continue to use thelegacy receiver until such time that the user decides to upgrade to areceiver, or box, that can recover the LL signal to provide theadditional services.

In a similar vein, a layered modulation based communication system canalso be used to provide an approach that is backward compatible. In alayered modulation based system at least two signals are modulated(again, e.g., a UL signal (legacy services) and an LL signal (additionalservices)) onto the same carrier (possibly asynchronously with eachother). Transmission of the UL signal and the LL signal occur separatelyvia two transponders and the front end of a layered modulation receivercombines them before recovery of the data transported therein.

SUMMARY OF THE INVENTION

We have observed that a receiver designed to receive and demodulatehierarchical modulation based signals cannot receive and demodulatelayered modulation based signals and vice versa. Thus, separatereceivers must be designed and inventoried for each respectivemodulation system. Therefore, and in accordance with the principles ofthe invention, a receiver includes a down converter for providing areceived signal and a demodulator having at least two demodulation modesfor demodulating the received signal, wherein one demodulation mode is ahierarchical demodulation mode and another demodulation mode is alayered demodulation mode.

In an embodiment of the invention, a satellite communications systemcomprises a transmitter, a satellite transponder and a receiver. Thetransmitter transmits an uplink multi-level modulated signal(hierarchical modulation or layered modulation) to the satellitetransponder, which broadcasts the multi-level modulated signal downlinkto one, or more, receivers. At least one receiver is capable ofoperating in any one of a number of demodulation modes for processing areceived signal. In particular, the receiver selects a demodulationprocess to perform as a function of the demodulation mode, wherein atleast two of the number of demodulation modes are a hierarchicaldemodulation mode and a layered demodulation mode; and the receiver thendemodulates the received signal in accordance with the selecteddemodulation mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an illustrative satellite communications system embodyingthe principles of the invention;

FIG. 2 shows an illustrative block diagram of a transmission paththrough satellite 15 of FIG. 1;

FIG. 3 shows an illustrative embodiment for implementing hierarchicalmodulation in transmitter 5 of FIG. 1;

FIG. 4 shows an illustrative symbol constellation for use in the upperlayer and the lower layer;

FIG. 5 shows an illustrative resulting symbol constellation for amulti-level signal;

FIG. 6 shows another illustrative embodiment for implementinghierarchical modulation in transmitter 5 of FIG. 1;

FIG. 7 shows an illustrative layered modulation embodiment for use intransmitter 5 of FIG. 1;

FIG. 8 shows an illustrative block diagram of a satellite transmissionpath in the context of a layered modulation based system;

FIG. 9 shows an illustrative block diagram of a receiver in accordancewith the principles of the invention;

FIG. 10 shows an illustrative block diagram of unifieddemodulator/decoder 320 of FIG. 9 in accordance with the principles ofthe invention;

FIGS. 11-15 show various blocks diagrams of different portions ofunified demodulator/decoder 320 in accordance with the principles of theinvention;

FIG. 16 shows an illustrative signal space;

FIG. 17 shows an illustrative log-likelihood look-up table in accordancewith the principles of the invention;

FIG. 18 shows an illustrative symbol constellation;

FIGS. 19 and 20 illustrate log-likelihood calculations;

FIG. 21 shows another variation of H-L mux 395 of FIG. 10;

FIGS. 22-23 show other illustrative embodiments of a unifieddemodulator/decoder in accordance with the principles of the invention;

FIG. 24 shows an illustrative flow chart in accordance with theprinciples of the invention; and

FIG. 25 shows another illustrative embodiment in accordance with theprinciples of the invention.

DETAILED DESCRIPTION

Other than the inventive concept, the elements shown in the figures arewell known and will not be described in detail. Also, familiarity withsatellite-based systems is assumed and is not described in detailherein. For example, other than the inventive concept, satellitetransponders, downlink signals, symbol constellations, a radio-frequency(rf) front-end, or receiver section, such as a low noise blockdownconverter, formatting and encoding methods (such as Moving PictureExpert Group (MPEG)-2 Systems Standard (ISO/IEC 13818-1)) for generatingtransport bit streams and decoding methods such as log-likelihoodratios, soft-input-soft-output (SISO) decoders, Viterbi decoders arewell-known and not described herein. In addition, the inventive conceptmay be implemented using conventional programming techniques, which, assuch, will not be described herein. Finally, like-numbers on the figuresrepresent similar elements.

An illustrative communications system 50 in accordance with theprinciples of the invention is shown in FIG. 1. Communications system 50includes transmitter 5, satellite channel 25, receiver 30 and television(TV) 35. Although described in more detail below, the following is abrief overview of communications system 50. Transmitter 5 receives anumber of data streams as represented by signals 4-1 through 4-K andprovides a multi-level modulated signal 6 to satellite transmissionchannel 25. Illustratively, these data streams represent controlsignaling, content (e.g., video), etc., of a satellite TV system and maybe independent of each other or related to each other, or a combinationthereof. The multi-level modulated signal 6 represents either ahierarchical modulation based signal or a layered modulation basedsignal having K layers, where K≧2. It should be noted that the words“layer” and “level” are used interchangeably herein. Satellite channel25 includes a transmitting antenna 10, a satellite 15 and a receivingantenna 20. Transmitting antenna 10 (representative of a groundtransmitting station) provides multi-level modulated signal 6 as uplinksignal 11 to satellite 15. Referring briefly to FIG. 2, an illustrativeblock diagram of the transmission path through satellite 15 for a signalis shown. Satellite 15 includes an input filter 155, a traveling wavetube amplifier (TWTA) 165 and an output filter 175. The uplink signal 11is first filtered by input filter 155, then amplified for retransmissionby TWTA 165. The output signal from TWTA 165 is then filtered by outputfilter 175 to provide downlink signal 16 (which is typically at adifferent frequency than the uplink signal). As such, satellite 15provides for retransmission of the received uplink signal via downlinksignal 16 to a broadcast area. This broadcast area typically covers apredefined geographical region, e.g., a portion of the continentalUnited States. Turning back to FIG. 1, downlink signal 16 is received byreceiving antenna 20, which provides a received signal 29 to receiver30, which demodulates and decodes received signal 29 in accordance withthe principles of the invention to provide, e.g., content to TV 35, viasignal 31, for viewing thereon. It should be noted that although notdescribed herein, transmitter 5 may further predistort the signal beforetransmission to compensate for non-linearities in the channel.

As noted above, in the context of this description multi-level modulatedsignal 6 represents either a hierarchical modulation based signal or alayered modulation based signal. In the case of the former, anillustrative block diagram for transmitter 5 is shown in FIG. 3; whilein the latter case an illustrative block diagram for transmitter 5 isshown in FIG. 7. In the remainder of this description it isillustratively assumed that there are two data streams, i.e., K=2. Itshould be noted that the invention is not limited to K=2 and, in fact, aparticular data stream such as signal 4-1 may already represent anaggregation of other data streams (not shown).

Turning first to FIG. 3, an illustrative hierarchical modulationtransmitter for use in transmitter 5 is shown. Hierarchical modulationis simply described as a synchronous modulation system where a lowerlayer signal is synchronously embedded into an upper layer signal so asto create a higher order modulation alphabet.

In FIG. 3, the hierarchical modulation transmitter comprises UL encoder105, UL modulator 115, LL encoder 110, LL modulator 120, multipliers (oramplifiers) 125 and 130, combiner (or adder) 135 and up converter 140.The upper layer (UL) path is represented by UL encoder 105, UL modulator115 and amplifier 125; while the lower layer (LL) path is represented byLL encoder 110, LL modulator 120 and amplifier 130. As used herein, theterm “UL signal” refers to any signal in the UL path and will beapparent from the context. For example, in the context of FIG. 3, thisis one or more of the signals 4-1, 106, 116 and 126. Similarly, the term“LL signal” refers to any signal in the LL path. Again, in the contextof FIG. 3, this is one of more of the signals 4-2, 111, 121 and 131.Further, each of the encoders implement known error detection/correctioncodes (e.g., convolutional or trellis codes; concatenated forward errorcorrection (FEC) scheme, where a rate ½, ⅔, ⅘ or 6/7code; LDPC codes(low density parity check codes); etc.). For example, typically ULencoder 105 uses a convolutional code or a short block code; while LLencoder 110 uses a turbo code or LDPC code. For the purposes of thisdescription it is assumed that LL encoder 110 uses an LDPC code. Inaddition, a convolutional interleaver (not shown) may also be used.

As can be observed from FIG. 3, signal 4-2 is applied to LL encoder 110,which provides an encoded signal 111 to LL modulator 120. Likewise,signal 4-1 is applied to UL encoder 105, which provides an encodedsignal 106 to UL modulator 115. Encoded signal 106 represents N bits persymbol interval T; while encoded signal 111 represents M bits per symbolinterval T, where N may, or may not, equal M. Modulators 115 and 120modulate their respective encoded signals to provide modulated signals116 and 121, respectively. It should be noted that since there are twomodulators, 115 and 120, the modulation can be different in the UL pathand the LL path. Again, for the purposes of this description it isassumed that the number of UL encoded data bits is two, i.e., N=2, andthat UL modulator 115 generates a modulated signal 116 that lies in oneof four quadrants of a signal space. That is, UL modulator 115 maps twoencoded data bits to one of four symbols. Similarly, the number of LLencoded data bits is also assumed to be two, i.e., M=2, and LL modulator120 also generates a modulated signal 121 that lies in one of fourquadrants of the signal space. An illustrative symbol constellation 89for use in both the UL and the LL is shown in FIG. 4. It should be notedthat signal space 89 is merely illustrative and that symbolconstellations of other sizes and shapes can be used.

However, the output signals from UL modulator 115 and LL modulator 120are further adjusted in amplitude by a predefined UL gain and apredefined LL gain via amplifiers 125 and 130, respectively. It shouldbe noted that the gains of the lower and upper layer signals determinethe ultimate placement of the points in the signal space. For example,the UL gain may be set to unity, i.e., 1, while the LL gain may be setto 0.5. The UL signal and the LL signal are then combined via combiner,or adder, 135, which provides combined signal 136. Thus, the modulatorof FIG. 3, e.g., the amplifiers 125 and 130, along with combiner 135,effectively further rearranges and partitions the signal space such thatthe UL signal specifies one of the four quadrants of the signal space;while the LL signal specifies one of a number of subquadrants of aparticular quadrant of the signal space as illustrated in FIG. 5 bysignal space 79.

In effect, the resulting signal space 79, also referred to herein as thecombined signal space 79, comprises 16 symbols, each symbol located at aparticular signal point in the signal space and associated with aparticular four bits. For example, symbol 83 is associated with the fourbit sequence “01 01”. The lower two bit portion 81 is associated withthe UL and specifies a quadrant of signal space 79; while the upper twobit portion 82 is associated with the LL and specifies a subquadrant ofthe quadrant specified by two bit portion 81. It should be noted thatsince the UL signal identifies the quadrant, the LL signal effectivelylooks like noise on the UL signal. In this regard, combined signal space79 is representative of the concept and the distances between symbolstherein is not to scale. Returning to FIG. 3, the combined signal 136 isapplied to up converter 140, which provides multi-level modulated signal6 at the appropriate transmission frequency. Turning briefly to FIG. 6,another illustrative embodiment for implementing hierarchical modulationin transmitter 5 is shown. FIG. 6 is similar to FIG. 3 except thathierarchical modulator 180 performs the mapping of the lower layer andupper layer bits into the combined signal space. For example, the upperlayer may be a QPSK (quadrature phase-shift keying) signal space, whilethe lower layer is a BPSK (binary phase-shift keying) signal space: inthis case, the resulting combined signal space would be, e.g., anon-uniform 8-PSK signal.

Turning now to FIG. 7, an illustrative block diagram of a layeredmodulator for use in transmitter 5 of FIG. 1 is shown. Here, while theelements of transmitter 5 are similar to those described above for FIG.3, transmitter 5 comprises two separate transmitter paths. The upperlayer path includes UL encoder 105, UL modulator 115 and up converter240. The lower layer path includes LL encoder 110, LL modulator 120 andup converter 245. Signal 4-1 is encoded by UL encoder 105 to provideencoded signal 106 representing N bits every upper layer symbolinterval, T_(UL), and signal 4-2 is encoded by LL encoder 110 to provideencoded signal 111 representing M bits every lower layer symbolinterval, T_(LL), where M may, or may not, be equal to N. The UL encodedsignal 106 is then modulated by UL modulator 115 to provide UL modulatedsignal 116, which is then upconverted to the appropriate frequency bandby up converter 240, which provides UL signal 6-1. Similarly, LL encodedsignal 111 is modulated by LL modulator 120 to provide LL modulatedsignal 121, which is then upconverted by up converter 245 to provide LLsignal 6-2. It should be observed from FIG. 7 that transmitter 5transmits two signals, i.e., multi-level modulated signal 6 comprises ULsignal 6-1 and LL signal 6-2. Typically, LL signal 6-2 is transmitted ata lower power level than UL signal 6-1. In fact, a layered modulationscheme typically requires careful power control between the upper layerpath and the lower layer path so that the recovery at the receiveroccurs in a meaningful manner.

As such, and referring now to FIG. 8, for a layered modulation basedsystem uplink signal 11 represents two uplink signals: UL uplink signal11-1 and LL uplink signal 11-2; while downlink signal 16 represents twodownlink signals: LL downlink signal 16-2 and UL downlink signal 16-1.In this example, satellite 15 of FIG. 1 may be a single satellite withtwo different transponders (one for the UL signal and the other for theLL signal) or two different satellites. Whether one satellite or two, asshown in FIG. 8 there are, in effect, two satellite transmission paths.The UL satellite path includes UL input filter 255, UL TWTA 265 and ULoutput filter 275, which provides UL downlink signal 16-1; while the LLsatellite path includes LL input filter 260, LL TWTA 270 and LL outputfilter 280, which provides LL downlink signal 16-2. Each of the elementsof FIG. 8 function in a similar fashion to the respective elements shownin FIG. 2 and described earlier.

As noted above, after reception of the downlink signal 16 by receivingantenna 20, receiver 30 demodulates and decodes received signal 29 toprovide, e.g., content to TV 35 for viewing thereon. An illustrativeportion of receiver 30 in accordance with the principles of theinvention is shown in FIG. 9. Receiver 30 includes front end filter 305,analog-to-digital converter 310 and unified demodulator/decoder 320.Front end filter 305 down-converts and filters received signal 29 toprovide a near base-band signal to A/D 310, which samples the downconverted signal to convert the signal to the digital domain and providea sequence of samples 311 (also referred to as multi-level signal 311)to unified demodulator/decoder 320. The latter, in accordance with theprinciples of the invention, has a number of demodulation modes, whereat least two of the demodulation modes represent a hierarchicaldemodulation mode and a layered demodulation mode. The selection of aparticular demodulation mode is provided by demodulation mode signal389, which is illustratively set a priori. Demodulation mode signal 389can be set in any one of a number of ways, e.g., a jumper setting,configuration information (not shown) of receiver 30 that may beviewable, e.g., on TV set 35, and settable, e.g., via a remote control(not shown), or from data transmitted on an out-of-band or an in-bandsignaling channel. If set in the hierarchical demodulation mode, unifieddemodulator/decoder 320 performs hierarchical demodulation ofmulti-level signal 311 and provides a number of output signals, 321-1 to321-K, representative of data conveyed by multi-level signal 311 on theK layers. Data from one or more of these output signals are provided toTV set 35 via signal 31. (In this regard, receiver 30 may additionallyprocess the data before application to TV set 35 and/or directly providethe data to TV set 35.) In the following example the number of levels istwo, i.e., K=2, but the inventive concept is not so limited. Forexample, in the hierarchical demodulation mode, unifieddemodulator/decoder 320 provides UL signal 321-1 and LL signal 321-2.The former ideally represents what was transmitted on the upper layer,i.e., signal 4-1 of FIG. 3; while the latter ideally represents what wastransmitted on the lower layer, i.e., signal 4-2 of FIG. 3. Similarly,if set in the layered demodulation mode, unified demodulator/decoder 320performs layered demodulation of multi-level signal 311 to provide ULsignal 321-1 and LL signal 321-2, which ideally represents signals 4-1and 4-2 of FIG. 7.

Turning now to FIG. 10, an illustrative block diagram of unifieddemodulator/decoder 320 is shown. Unified demodulator/decoder 320comprises UL demodulator 330, delay/equalizer element 345, UL decoder335, UL remodulator/reencoder 350, combiner 375, LL demodulator 390, H-Lmultiplexer (H-L mux) 395 (also referred to herein as H-L selector 395)and LL decoder 340. Multi-level signal 311 is applied to UL demodulator330, which demodulates this signal and provides therefrom a UL carriersignal 332, a resampled multi-level signal 316 and a demodulated ULsignal as represented by demodulated UL signal point stream 333.Referring now to FIG. 11, an illustrative block diagram of ULdemodulator 330 is shown. UL demodulator 330 includes digital resampler415, matched filter 420, derotator 425, timing recovery element 435 andcarrier recovery element 440. Multi-level signal 311 is applied todigital resampler 415, which resamples multi-level signal 311 using ULtiming signal 436, which is provided by timing recovery element 435, toprovide resampled multi-level signal 316. Resampled multi-level signal316 is applied to matched filter 420 and is also provided todelay/equalizer element 345 (described below). Matched filter 420 is aband-pass filter for filtering resampled multi-level signal 316 aboutthe UL carrier frequency to provide a filtered signal to both derotator425 and the above-mentioned timing recovery element 435, which generatestherefrom UL timing signal 436. Derotator 425 derotates, i.e., removesthe carrier from the filtered signal to provide a demodulated UL signalpoint stream 333. Carrier recover element 440 uses the demodulated ULsignal point stream 333 to recover therefrom UL carrier signal 332,which is applied to derotator 425 and to UL remodulator/reencoder 350(described below).

Referring back to FIG. 10, UL decoder 335 acts in a complementaryfashion to corresponding UL encoder 105 of transmitter 5 and decodes thedemodulated UL signal point stream 333 to provide UL signal 321-1. Asnoted above, UL signal 321-1 represents the data conveyed on the upperlayer, e.g., as represented by signal 4-1 of FIGS. 3 and 7. It should beobserved that UL decoder 321-1 recovers the data conveyed in the UL by,in effect, treating the LL signal as noise on the UL signal. In otherwords, UL decoder 335 operates as if UL signal 321-1 represents symbolsselected from signal space 89 of FIG. 4. As an alternative to rerotatingthe reconstructed signal for subtraction, the combined signal can bederotated for subtraction.

UL signal 321-1 is also applied to remodulator/reencoder 350, which,responsive to UL carrier signal 332, locally reconstructs the ULmodulated signal. In particular, remodulator/reencoder 350 reencodes andthen remodulates UL signal 321-1 to provide UL modulated signal 351 to anegative input terminal of combiner 375. Referring briefly to FIG. 12, ablock diagram of an illustrative remodulator/reencoder 350 is shown.Remodulator/reencoder 350 includes rotate phase delay element 445,encoder 470, rerotator 465 and pulse shaping element 460. Encoder 470reencodes and maps UL signal 321-1 to provide an encoded symbol stream471 to rerotator 465, which re-rotates encoded symbol stream 471 by adelayed version of the locally generated UL carrier frequency, asdetermined by the upper layer carrier recovery element 440. The outputsignal from rerotator 465 is applied to pulse shaping element 460, whichfurther shapes the reconstructed signal to provide UL modulated signal351.

Turning back to FIG. 10, combiner 375 subtracts UL modulated signal 351from a delayed and equalized version (signal 346) of resampledmulti-level signal 316 to provide a signal representative of just thereceived LL modulated signal, i.e., LL modulated signal 376, which isalso used to update taps (not shown) of the equalizer of delay/equalizerelement 345. The two input signals to the combiner 375 are at the samesampling rate, typically an integer multiple of the upper layer symbolrate. An illustrative block diagram of delay/equalizer element 345 isshown in FIG. 13. Delay/equalizer element 345 includes signal delayelement 450 and equalizer 455. Signal delay element 450 compensates forthe delay in the signal processing path through UL demodulator 330,decoder 335 and remodulator/reencoder 350; while equalizer 455 attemptsto remove linear distortions, such as tilts on the signal path in thetuner, such that combiner 375, in effect, cancels as much of the ULsignal as possible from the resampled multi-level signal 316 to providea clean LL modulated signal 376. In other words, equalization isperformed to optimally match the UL component of resampled multi-levelsignal 316 to locally reconstructed UL modulated signal 351 so as tooptimally remove the UL signal before demodulating and decoding the LLsignal.

Returning again to FIG. 10, LL modulated signal 376 is then applied toLL demodulator 390, which recovers therefrom a demodulated LL signal asrepresented by demodulated LL signal point stream 391. An illustrativeblock diagram of LL demodulator 390 is shown in FIG. 14. LL demodulator390 includes digital resampler 515, matched filter 520, timing recoveryelement 535, derotator 525, and carrier recovery element 540. LLmodulated signal 376 is applied to digital resampler 515, whichresamples LL modulated signal 376 using LL timing signal 536 to bringthe LL signal to the initial LL processing rate, which is typically aninteger multiple of the lower layer symbol rate. Digital resampler 515works in conjunction with timing recovery element 535. Resampled LLmodulated signal 516 is applied to matched filter 520, which is aband-pass filter for filtering and shaping resampled LL modulated signal516 about the LL carrier frequency to provide a filtered signal to bothderotator 525 and the above-mentioned timing recovery element 535, whichgenerates therefrom LL timing signal 536. Derotator 525 derotates, i.e.,removes the carrier from the filtered signal to provide a demodulated LLsignal point stream 391, which is also applied to carrier recoverelement 540. The latter uses the demodulated LL signal point stream 391to provide a recovered LL carrier signal to derotator 525.

Returning once again to FIG. 10, H-L mux 395 receives demodulated ULsignal point stream 333 and demodulated LL signal point stream 391. Inaccordance with the principles of the invention, H-L mux 395 selectseither UL signal point stream 333 or LL signal point stream 391 forprocessing and subsequent application to LL decoder 340 as a function ofdemodulation mode signal 389. If demodulation mode signal 389 indicateslayered demodulation, then H-L mux 395 selects LL signal point stream391 for processing. However, if demodulation select signal 389 indicateshierarchical demodulation, then H-L mux 395 selects UL signal pointstream 333 for processing.

Attention should now be directed to FIG. 15, which shows an illustrativeblock diagram of H-L mux 395. The latter comprises multiplexer (mux) 565and log-likelihood ratio (LLR) look-up table (LUT) 570. The inputsignals to H-L mux 395 are received signal point values (either from theUL or the LL) and the output signals of H-L mux 395 are soft valuesrepresenting the probability that certain bits were received. Inparticular, Mux 565 selects either UL signal point stream 333 or LLsignal point stream 391 as a function of demodulation mode signal 389,as described above, and provides the selected signal as received signal566. As such, received signal 566 is a stream of received signal points,each received signal point having an in-phase (I_(REC)) component (572)and a quadrature (Q_(REC)) component (571) in a signal space. This isfurther illustrated in FIG. 16 for a received signal point Z_(REC),where:z=I _(rec) +j Q _(rec).  (1)

The I_(REC) and Q_(REC) components of each received signal point areapplied to LLR LUT 570. The latter stores a LUT 599 of precomputed LLRvalues as illustrated in FIG. 17. In particular, each row of LUT 599 isassociated with a particular I component value (an I row value), whileeach column of LUT 599 is associated with a particular Q component value(a Q column value). LUT 599 has L rows and J columns. LLR LUT 570quantizes the IREC and QREC component values of a received signal pointof received signal 566 to form an input address, which is used as anindex into LUT 599 for selecting therefrom a respective precomputed LLR.Each lower-layer symbol interval, T_(LL), the selected LLR is providedvia signal 396 to LL decoder 340. For example, if the I_(REC) componentvalue of signal 566 is quantized to the first row and the Q_(REC)component value of signal 566 is quantized to the first column, then LLR598 would be selected and provided via signal 396 of FIG. 15 to LLdecoder 340 of FIG. 10.

Other than the inventive concept, and as known in the art, for a givenbit-to-symbol mapping M(b_(i)), where M are the target symbols and bi=0,1, . . . B−1, are the bits to be mapped where B is the number of bits ineach symbol (e.g., B may be two bits for QPSK, three bits for 8-PSK,etc.), the log-likelihood ratio function for the ith bit of a B bitvalue is:LLR(i, z)=log [(prob(b _(i)=1|z))/(prob(b _(i)=0|z))];   (2)where b_(i) is the ith bit and z is the received signal point in thesignal space. The notation “prob (b_(i)=1|z)” represents the probabilitythat the ith bit is a “1” given that the signal point z was received.Similarly, the notation “prob (b_(i)=0|z)” represents the probabilitythat the ith bit is a “0” given that the signal point z was received.

For a two-dimensional signal space, the probabilities within equation(2) are assumed to be based upon additive Gaussian white noise (AWGN)having a probability density function (PDF) of:

$\begin{matrix}{{{prob}(n)} = {\frac{\exp\left( \frac{- {n}^{2}}{2\sigma^{2}} \right)}{2{\pi\sigma}^{2}}.}} & (3)\end{matrix}$Therefore, the LLR for a given bit and received signal point are definedas:

$\begin{matrix}{{{LLR}\left( {i,z} \right)} = {{\log\left\lbrack \frac{\sum\limits_{M_{{{bh}\mspace{11mu} i} = 1}}{\exp\left( \frac{- {{z - M}}^{2}}{2\sigma^{2}} \right)}}{\sum\limits_{M_{{{bh}\mspace{11mu} i} = 0}}{\exp\left( \frac{- {{z - M}}^{2}}{2\sigma^{2}} \right)}} \right\rbrack}.}} & (4)\end{matrix}$It can be observed from equation (4) that the LLR for a given receivedsignal point z is a function of z, the target symbols M, and the rmsnoise level σ. An LLR is also one example of a “soft metric.”

A pictorial illustration of the calculation of an LLR ratio is shown inFIGS. 18 and 19. FIG. 18 shows an illustrative LL symbol constellation.For simplicity a 4 symbol QPSK (quadrature phase shift keyed)constellation is shown, however, it should be noted that other sizes andshapes of symbol constellations could also have been used, e.g., 3 bitsfor 8-PSK, 4 bits for 16-QAM, a hierarchical 16-QAM, etc. As can beobserved from FIG. 18, there are four symbols in the signal space 89,each symbol associated with a particular two bit mapping [b1, b0].Turning now to FIG. 19, a received signal point z is shown in relationto the symbols of signal space 89. It can be observed from FIG. 19 thatthe received signal point z is located at different distances d_(i) fromeach of the symbols of signal space 89. For example, the received signalpoint z is located a distance d₄ from the symbol associated with the twobit mapping “01.” As such, the LLR(b0) is:ln[(probability b0 is one)/(probability b0 is zero)]; or  (5A)ln[(probability (symbol 01 or 11))/(probability (symbol 00 or 10))];or  (5B)ln[{exp(−d₄ ²/(2σ²))+exp(−d₃ ²/(2σ²))}/{exp(−d₂ ²/(2σ²))+exp(−d₁²/(2σ²))}].  (5C)while the LLR(b1) is:ln[(probability b1 is one)/(probability b1 is zero)]; or  (6A)ln[(probability (symbol 10 or 11))/(probability (symbol 00 or 01))];or  (6B)ln[{exp(−d₁ ²/(2σ²))+exp(−d₃ ²/(2σ²))}/{exp(−d₂ ²/(2σ²))+exp(−d₄²/(2σ²))}].  (6C)

Returning to FIG. 15, it can be observed that LLR LUT 570 (i.e., LUT599) is initialized to either a set of hierarchical LLR values 573 orlayered LLR values 574 depending on the respective mode of receiver 30.For example, the layered LLR values are calculated a priori with respectto a LL symbol constellation such as illustrated in FIGS. 4, 18 and 19;while the hierarchical LLR values are calculated a priori with respectto the combined symbol constellation such as the one illustrated in FIG.5 and shown again in FIG. 20. In other words, the hierarchical LLRs forthe LL are determined—not with respect to the LL signal space (e.g.,signal space 89 of FIG. 4)—but with respect to the combined signal space(e.g., signal space 79 of FIG. 5). For every received signal point z, adistance between each symbol of signal space 79 and the received signalpoint z is determined and used in calculating an LLR. For simplicity,only some of these distances, d_(i), are shown in FIG. 20. Thehierarchical LLR values 573 and the layered LLR values 574 can be formedin any number of ways. For example, receiver 30 may perform thecalculations by using, e.g., a training signal, provided by transmitter5 either during start-up, or re-initialization, of communicationsbetween the two endpoints (transmitter 5 and receiver 30). As known inthe art, a training signal is a predefined signal, e.g., a predefinedsymbol sequence that is known a priori to the receiver. A predefined“handshaking” sequence may further be defined, where the endpointsexchange signaling before communicating data therebetween.Alternatively, the calculations may be performed remotely, e.g., at thelocation of transmitter 5 and sent to receiver 30 via an in-band orout-of-band signaling channel (this could even be via a dial-up facility(wired and/or wireless) (not shown)). Alternatively, the calculationscan be performed analytically and the hierarchical LLR values 573 orlayered values 574 preprogrammed into a memory at the time ofmanufacture of the receiver.

Referring back to FIG. 10, LL decoder 340 receives the sequence of LLRs(the soft input data), via signal 396, and provides therefrom LL signal321-2. LL decoder 340 operates in a complementary fashion to that of LLencoder 110. It should also be noted that LL decoder 340 may also be asoft-input-soft-output decoder, and provide soft output values, whichare then additionally processed (not shown) to form LL signal 321-2.

Thus, in a layered demodulation mode, and as can be observed from FIG.10, receiver 30 sequentially demodulates the received signal by firstrecovering the UL signal via UL demodulator 330 and decoder 335. Therecovered UL signal is then reencoded and remodulated for subtractionfrom the received signal to uncover the LL signal for demodulation by LLdemodulator 390. The resulting demodulated LL signal point stream 391 isthen processed to generate soft input data, e.g., LLRs, with respect tothe LL symbol constellation. In contrast, in a hierarchical demodulationmode, the UL signal point stream 333 is recovered, from which the LLsignal is then directly determined. This is referred to herein as asimultaneous mode of decoding. In particular, the UL signal point stream333 is processed to generate soft input data, e.g., LLRs, to recovertherefrom the LL data.

Other variations of H-L mux 395 are possible. For example, FIG. 21 showsan illustration where two separate look-up tables (555 and 560) arelocated in front of mux 565, which selects the appropriate signal(either signal 556 or 561) in accordance with demodulation mode signal389.

Another embodiment in accordance with the principles of the invention isshown in FIG. 22. Illustratively, in this embodiment a unifieddemodulator/decoder 320′ sequentially decodes the received signal whenin the hierarchical mode of operation. For sequential decoding of ahierarchical modulation based signal, the receiver first decodes the ULsignal and then decodes the LL signal. As can be observed from FIG. 22,unified demodulator/decoder 320′ is similar to unifieddemodulator/decoder 320 of FIG. 10 except for the addition of combiner,or adder 380, delay element 355 and H-L mux 395′. Delay element 355compensates for the processing delay of UL decoder 335, encoder 470,etc. Illustratively, adder 380 receives as input signals the delayeddemodulated UL signal point stream 333′ and symbol stream 471, which isavailable from UL remodulator/reencoder 350 as shown in FIG. 12.Combiner 380 subtracts the encoded symbol stream 471 from delayeddemodulated UL signal point stream 333′ to provide an LL signal pointstream 381 to an input of H-L mux 395′. As before, H-L mux 395′ selectsthe applied signals, here, either LL signal point stream 381 or thedemodulated LL signal point stream 391 as a function of the selecteddemodulation mode.

A block diagram of H-L mux 395′ is shown in FIG. 23. In this example,H-L mux 395′ includes mux 565 and LLR calculator 580. Mux 565 selectsbetween LL signal point stream 381 or the demodulated LL signal pointstream 391 as a function of demodulation mode signal 389 to providereceived signal point stream 566. The latter is applied to a soft datagenerator, such as represented by LLR calculator 580, which provides LLRdata 396 to LL decoder 340, as described above.

Attention should now be directed to FIG. 24, which shows an illustrativeflow chart in accordance with the principles of the invention of aprocess for use in receiver 30 of FIG. 1. In step 605, receiver 30selects a one of a number of demodulation modes. Illustratively, thereare at least two demodulation modes: hierarchical demodulation andlayered demodulation. As noted above, this selection can be performedby, e.g., a jumper setting, a configuration screen (not shown) ofreceiver 30, or from data transmitted on an out-of-band or an in-bandsignaling channel. In step 610, receiver 30 receives a multi-levelsignal. In step 615, receiver 30 determines the demodulation process toperform as a function of the selected demodulation mode. If thedemodulation mode is hierarchical, then receiver 30 performshierarchical demodulation of the received multi-level signal in step620. On the other hand, if the mode of demodulation is layered, thenreceiver 30 performs layered demodulation of the received multi-levelsignal in step 625. It should be noted that selection of thedemodulation mode (step 605) may be performed after receiving themulti-level signal (step 610).

Another illustrative embodiment of the inventive concept is shown inFIG. 25. However, only those portions relevant to the inventive conceptare shown. For example, analog-digital converters, filters, decoders,etc., are not shown for simplicity. In this illustrative embodiment anintegrated circuit (IC) 705 for use in a receiver (not shown) includesunified demodulator/decoder 320 and at least one register 710, which iscoupled to bus 751. The latter provides communication to, and from,other components of the receiver as represented by processor 750.Register 710 is representative of one, or more, registers of IC 705,where each register comprises one, or more, bits as represented by bit709. The registers, or portions thereof, of IC 705 may be read-only,write-only or read/write. In accordance with the principles of theinvention, unified demodulator/decoder 320 decodes a receivedmulti-level modulated signal and at least one bit, e.g., bit 709 ofregister 710, is a programmable bit that can be set by, e.g., processor750, for controlling the operation of unified demodulator/decoder 320.In the context of FIG. 16, IC 705 receives an IF signal 701 forprocessing via an input pin, or lead, of IC 705. A derivative of thissignal, 311, is applied to unified demodulator/decoder 320. The latterprovides output signals 321-1 through 321-K as described above. Unifieddemodulator/decoder 320 is coupled to register 710 via internal bus 711,which is representative of other signal paths and/or components of IC705 for interfacing unified demodulator/decoder 320 to register 710 asknown in the art.

As described above, and in accordance with the inventive concept, areceiver handles both hierarchical modulation and layered modulation ina unified framework. Although only two demodulation modes were describedherein, the inventive concept is not so limited and, as such, a receiverin accordance with the principles of the invention may have more thantwo demodulation modes. It should be noted that although the inventiveconcept was described in the context of LL decoder 340 receiving softmetrics, LL decoder 340 may receive signal points and, as such, furtherprocess the received signal point data to derive therefrom LLRs asdescribed above. In this context, the above-described H-L mux element issimply a multiplexer for selecting the received signal point stream suchas mux 565 of FIG. 23. It should also be noted that although describedin the context of a receiver coupled to a display as represented by TV35, the inventive concept is not so limited. For example, receiver 30may be located further upstream in a distribution system, e.g., at ahead-end, which then retransmits the content to other nodes and/orreceivers of a network. Further, although hierarchical modulation andlayered modulation were described in the context of providingcommunication systems that are backward compatible, this is not arequirement of the inventive concept. It should also be noted thatgroupings of components for particular elements described and shownherein are merely illustrative. For example, either or both UL decoder335 and LL decoder 340 may be external to element 320, which then isessentially a demodulator that provides at least a demodulated upperlayer signal and a demodulated lower layer signal.

As such, the foregoing merely illustrates the principles of theinvention and it will thus be appreciated that those skilled in the artwill be able to devise numerous alternative arrangements which, althoughnot explicitly described herein, embody the principles of the inventionand are within its spirit and scope. For example, although illustratedin the context of separate functional elements, these functionalelements may be embodied on one or more integrated circuits (ICs).Similarly, although shown as separate elements, any or all of theelements of may be implemented in a stored-program-controlled processor,e.g., a digital signal processor (DSP) or microprocessor that executesassociated software, e.g., corresponding to one or more of the stepsshown in FIG. 24. Further, although shown as separate elements, theelements therein may be distributed in different units in anycombination thereof. For example, receiver 30 may be a part of TV 35. Itis therefore to be understood that numerous modifications may be made tothe illustrative embodiments and that other arrangements may be devisedwithout departing from the spirit and scope of the present invention asdefined by the appended claims.

1. A receiver comprising: a down convener for providing a receivedsignal; and a demodulator having at least two demodulation modes fordemodulating the received signal, wherein one demodulation mode ishierarchical demodulation and another demodulation mode is layereddemodulation, and wherein the demodulator comprises an upper layerdemodulator for processing the received signal to provide a demodulatedupper layer signal; an upper layer decoder for decoding the demodulatedupper layer signal to provide a decoded upper layer signal; an upperlayer remodulator/reencoder responsive to the decoded upper layer signalfor providing a reconstructed modulated upper layer signal; a combinerfor combining the received signal with the reconstructed modulated upperlayer signal such that an upper layer signal component of the receivedsignal is substantially reduced therefrom to provide a received lowerlayer signal; a lower layer demodulator for processing the receivedlower layer signal to provide a demodulated lower layer signal; aselector for providing a lower layer signal derived from either thedemodulated lower layer signal or the demodulated upper layer signal;and a lower layer decoder for decoding the lower layer signal to providea decoded lower layer signal.
 2. The receiver of claim 1, wherein thedemodulator is responsive to a demodulation mode signal that specifieswhich one of the number of demodulation modes is performed by thedemodulator.
 3. The receiver of claim 1, wherein the selector isresponsive to a demodulation mode signal for selecting either thedemodulated lower layer signal or the demodulated upper layer signal foruse in deriving the lower layer signal.
 4. The receiver of claim 3,wherein the selector is responsive to the demodulation mode signal forselecting one of a number of log-likelihood ratio (LLR) look-up tablesfor use in deriving the lower layer signal.
 5. The receiver of claim 1,further including an equalizer deposed between the received signal andthe combiner for equalizing the received signal.
 6. A receivercomprising: a down converter for providing a received signal; and ademodulator having at least two demodulation modes for demodulating thereceived signal, wherein one demodulation mode is hierarchicaldemodulation and another demodulation mode is layered demodulation,wherein the demodulator comprises: an upper layer demodulator forprocessing the received signal to provide a demodulated upper layersignal; an upper layer decoder for decoding the demodulated upper layersignal to provide a decoded upper layer signal; an upper layerremodulator/reencoder responsive to the decoded upper layer signal forproviding a reconstructed modulated upper layer signal and areconstructed encoded upper layer signal; a combiner for combining thereceived signal with the reconstructed modulated upper layer signal suchthat an upper layer signal component of the received signal issubstantially reduced therefrom to provide a received lower layersignal; a combiner for combining the demodulated upper layer signal andthe reconstructed encoded upper layer signal such that an upper layersymbol component of the demodulated upper layer signal is substantiallyreduced to provide a first demodulated lower layer signal; a lower layerdemodulator for processing the received lower layer signal to provide asecond demodulated lower layer signal; a selector for providing a lowerlayer signal derived from either the first demodulated lower layersignal or the second demodulated lower layer signal; and a lower layerdecoder for decoding the lower layer signal to provide a decoded lowerlayer signal.
 7. The receiver of claim 6, wherein the selector isresponsive to a demodulation mode signal for selecting either thedemodulated lower layer signal or the demodulated upper layer signal foruse in deriving the lower layer signal.
 8. The receiver of claim 7,wherein the selector further includes a soft input generator forconvening the selected signal into soft input data, which is thenprovided as the lower layer signal.
 9. The receiver of claim 8, whereinthe soft input generator is a log-likelihood ratio generator.
 10. Thereceiver of claim 6, further including an equalizer deposed between thereceived signal and the combiner for combining the received signal, forequalizing the received signal.
 11. A method for use in a receiver, themethod comprising: receiving a signal; selecting one of a number ofdemodulation modes, wherein at least two of the number of demodulationmodes are a hierarchical demodulation mode and a layered demodulationmode; and demodulating by a demodulator the received signal inaccordance with the selected demodulation mode; wherein the demodulatingstep includes the steps of: demodulating the received signal to providea demodulated upper layer signal and a demodulated lower layer signal;decoding the demodulated upper layer signal to provide a decoded upperlayer signal; selecting, as a function of the selected demodulationmode, either the demodulated lower layer signal or the demodulated upperlayer signal for providing a lower layer signal, wherein the demodulatedlower layer signal is selected when the demodulation mode is the layereddemodulation mode and the demodulated upper layer signal is selectedwhen the demodulation mode is the hierarchical demodulation mode; anddecoding the lower layer signal to provide a decoded lower layer signal.12. The method of claim 11, wherein the selecting step includes thesteps of: selecting a log-likelihood ratio (LLR) look-up table (LUT) asa function of the demodulation mode signal; and generatinglog-likelihood ratios from the LLR LUT as a function of the selectedsignal to provide the lower layer signal.
 13. A method for use in areceiver, the method comprising: receiving a signal; selecting one of anumber of demodulation modes, wherein at least two of the number ofdemodulation modes are a hierarchical demodulation mode and a layereddemodulation mode; and demodulating by a demodulator the received signalin accordance with the selected demodulation mode; wherein thedemodulating step includes the steps of: demodulating the receivedsignal to provide a demodulated upper layer signal and a demodulatedlower layer signal; decoding the demodulated upper layer signal toprovide a decoded upper layer signal; reencoding the decoded upper layersignal to provide a reencoded upper layer signal; subtracting thereencoded upper layer signal from the demodulated upper layer signal toprovide an encoded lower layer signal; selecting, as a function of theselected demodulation mode, either the demodulated lower layer signal orthe encoded lower layer signal for providing a lower layer signal,wherein the demodulated lower layer signal is selected when thedemodulation mode is the layered demodulation mode and the encoded lowerlayer signal is selected when the demodulation mode is the hierarchicaldemodulation mode; and decoding the lower layer signal to provide adecoded lower layer signal.
 14. The method of claim 13, wherein theselecting step includes the step of generating log-likelihood ratiosfrom the selected signal for providing the lower layer signal.